Burn-off ovens work with combustible atmosphere in a pyrolytic, or oxygen deprived process to remove organic waste. A "charge" of materials for processing is placed in an insulated primary chamber which is fired from the bottom, with combustion gases and vaporized volatile constituents being carried off by an exhaust stack at the top of the oven. A circulation pattern in the oven interior is initiated by burner discharge flow, passes through and around the charge, is continued by convection and completed by the low pressure of exhaust stack draft and/or aspiration into an afterburner system. Bottom firing is a child of the hearth and the baking oven, refined over the years, and is ideal for most processes. Waste incineration, for example, where the heat of bottom firing rises to preheat the charge and where the burning of underneath materials helps kindle the top loaded material, is well served by this practice.
The earliest burn-off furnace rightfully deserved its name, when organic materials such as paint or oil were literally burned from metal parts. From this beginning evolved the present pyrolytic oven, wherein organic materials are vaporized in an oxygen poor atmosphere to leave only the inorganic ash, without combustion. In this method, parts being cleaned are not subjected to such high temperatures as they were previously. Afterburners, which were developed for use on incinerators, were soon added for control of smoke and odor, arriving at the configuration of the accepted, present day burn-off oven.
Practitioners of the art have given us improved methods for control of pyrolytic ovens, as disclosed by Kelly, U.S. Pat. No. 4,270,989; Mainord, U.S. Pat. No. 4,557,203; Koptis, et al, U.S. Pat. Nos. 4,759,298 & 4,827,855; and the present inventor, Mann, U.S. Pat. No. 5,189,963. However, in each of the foregoing, it is to be noted that oven configuration has remained essentially static, so that, aside from control methodology, there is little to distinguish one bottom fired burn-off oven from the next.
Top firing, although known and used on some curing and heat treating ovens, has heretofore been been considered inappropriate for use in burn-off ovens by practitioners of the art. The bottom fired process has been seen as a synergy of "rising heat" while top firing is thought of as ineffective and unsuited to burn-off. Because both burn-off ovens and incinerators are fired waste disposal processes with stringent emission controls, they have been viewed as closely related, in spite of the unique nature of pyrolysis. In truth, burn-off oven design has evolved from incinerator practice more so than as an independent and unrelated art.
The pyrolytic process has thus, been made to work under compromising conditions. Hot combustion gases, at up to 1,200.degree. F., play directly on parts being cleaned and on the carts carrying them, risking parts damage and maintenance problems. Also, concentrations of dense combustible gases tending to collect in undisturbed pockets inside the oven complicate control and pose a lingering threat of fire or explosion. Thus, prudence causes us to control the process slowly and carefully, while surplus heat goes up the exhaust stack. Objects of the present invention therefore, are to provide method and means for conducting pyrolytic cleaning processes in a readily controlled manner, with minimized risk of damage to the contents, with better energy efficiency and improved safety.
These objects are achieved in the present invention by literally turning the process upside-down; introducing hot combustion gases at the upper level of the oven chamber and exhausting cooler gases from the lower level. The hot gases rise and spread out over the length and width of the oven and, as the top of the chamber is heated, the gases cool and descend. The charge is progressively heated from above as the temperature builds downwardly and pyrolysis of the organic materials proceeds accordingly. Progressive pyrolysis makes fire and explosion suppression less demanding, in clear contrast to the result of direct firing from underneath, where heat rises through the charge. The control system is adapted to "top-down" oven characteristics by placing a thermocouple at the chamber top, the hottest location in the oven, for control of the primary burner and/or water cooling. Another thermocouple, preferably placed at the chamber bottom, the coolest location in the oven, may be used to assist in controlling duration of the process. The temperature differential between the top and bottom is greatest when heating starts and decreases as oven heating progresses, reaching a minimum at the stable, or "steady state", fully heated condition. Another reason that others have considered "top-down" inappropriate for burn-off ovens is the characteristic of frequently cutting back the primary burner for cooling as the oven heats, making the process seem too slow and inefficient.
The top-down heating process has produced unanticipated advantages. Ash released into the atmosphere is reduced, a result of not exposing the charge directly to the velocity of burner gases, and oven temperature control is smoother and more predictable. This is ascribed both to the ease with which the top mounted cooling spray reaches the hottest gases in the oven and also to permitting a natural, downward flow of the relatively dense organic vapors. The downward flow takes organic vapors away as they are formed, to be processed steadily and smoothly in the afterburner. The exothermic energy contributed by these organic vapors had been considered to be a plus for process efficiency in bottom fired ovens but now, taking the energy directly to the afterburner has proven to increase overall efficiency as well as reduce parts damage.